1. Technical Field
The present invention relates generally to analog integrated circuits and more specifically to current amplification including voltage-to-current conversion and current amplification.
2. Discussion of the Prior Art
While there are many standard circuits available for voltage amplification, current amplification circuits are uncommon. In a typical current amplifier, amplification is achieved through the use of a current mirror with different emitter areas for each transistor and different emitter resistors. At large values of gain, precise matching of emitter areas and resistance becomes critical.
Referring to FIG. 1, the classical manner of amplifying the current is shown. The current amplifier of FIG. 1 includes current source I1, transistors Q1, Q2, and resistive elements R1, R2. The transistors labeled Q1 and Q2 have differing emitter areas of 1 unit and 2 units, respectively. In all other aspects transistors Q1 and Q2 are identical. Because of the difference in emitter areas, the saturation current out of Q2 will be twice that of Q1; IS2=2*IS1.
The voltage between the base and ground is given by:Vb=I1*R1+VT*In(I1/Is1)  (1) 
Vb may also be found as:Vb=I2*R2+VT*In(I2/Is2)  (2) 
Equating (1) and (2), we have:I1*R1+VT*In(I1/Is1)=I2*R2+VT*In(I2/Is2)  (3) with R1=2*R2, and Is2=2*Is1
Equation (3) has the solution I2=2*I1, and so the output current is twice the input current. This output gain is determined by the choice of resistive elements R1 and R2, in conjunction with the difference in emitter areas.
This circuit has several drawbacks. For large values of current gain, the resistive elements R1 and R2 and the emitter elements of transistors Q1 and Q2 must be matched very accurately, as can be seen by inspection of equation (3). A further drawback of this circuit is that the values of gain that may be produced are limited to the relative values of the resistors and the emitter areas. This gain value is restricted to integer values.
A circuit with a simple structure, accepting either a current source or voltage source as an input, and producing identical current outputs could be used to address the shortcomings of the current amplifier prior art. Also, it would be advantageous if such a circuit could be used as a standard current amplifier cell in a variety of applications in which current amplification or voltage-to-current conversion is needed.